[SI-LIST] Re: Importance of Package Height
- From: "Abe Riazi" <ARIAZI@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sat, 9 Mar 2002 08:12:56 -0800
John Barnes Wrote:
>I found
> that a fair model for the ESL of a ceramic capacitor is to treat it as a
> non-ferrous blob of the same size. From theory, the partial inductance
> of a non-ferrous strap of length=L, width=W, and thickness=T is:
>
> ESL = (muv * L)/(2 * pi) *
> [ ln(2 * L / (W + T)) +0.5 + (0.2235 * (W + T) / L) ] henries,
>
> where muv is the permeability of vacuum or about 1.257 uH/meter.
>
> So to a first approximation, when the thickness of the capacitor is
> close to its width, the ESL will be roughly proportional to the
> capacitor's length.
>
Hi John:
Formulas for calculating ESL are interesting to me. I have also seen
following equations published by Doug Brooks (in "ESR and Bypass
Capacitor Self Resonant Behavior How to Select Bypass Caps", Page 4):
C( i ) = (0.1 * ((0.01)^(( i-1 )/(nvalues -1)))) * 10^(-6)
L( i ) = (10 * ((0.01)^(( i-1 )/(nvalues - 1)))) * 10^(-9)
Where, i ranges from 1 to nvalues (number of capacitor values).
A notable outcome is that the ratio of inductance L (in Henries)
to capacitance C (in Farads) yields a simple constant:
L( i )/C( i ) = [10 * 10^(-9)]/[0.1* 10^(-6)] = 10^(-8)/10^(-7) = 0.1
This implies (under those conditions when above relations are valid)
knowing C allows calculating L.
Examples.
If C = 0.100 uF, then L = 10 nH;
If C = 0.001 uF, then L = 0.1 nH.
The equation you have given ( which relates ESL to capacitor
dimensions ) and those by Doug Brooks ( relating ESL to capacitor
value ) are useful; but also require understanding their application
domains/limits.
Is it correct to conclude that the only limitation/requirement to your
formula is for the capacitor to be surface mount ceramic?
Thank you .
Abe Riazi
ServerWorks
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